Pressure-sensitive switches are used in a variety of applications where it is desired to switch apparatus on or off at predetermined pressures. Switching may be desirable, for example, because the electrical circuitry controlled by the switch may exceed its design limits, might be damaged, or give inaccurate and misleading readings when operated at, extreme pressures. Pressure-sensitive switches are also required by certain government regulations in commercial forms of apparatus capable of both commercial and military uses to prevent commercial forms of the apparatus from being converted to military applications.
One important application of pressure-sensitive switches is in hydrophone streamer cable arrays used in underwater surveying. In such surveying, a survey ship tows a plurality of submerged cables extending substantially perpendicular to the ship's direction of travel. Each of the plurality (typically 4-10) of hydrophone cables is secured to one of a series of laterally spaced apart drums located on the ship stem to keep them laterally separated so that they extend parallel to each other and to the ship's direction of travel. Additional lateral control is provided by paravanes associated with each cable to steer them as necessary. These hydrophone cables are of substantial length, up to 5000 meters. Each cable comprises a waterproof hollow elongate prismatic sheath, typically a hollow, flexible polymeric tube and at least one tensile member fixedly associated with the sheath; this tensile member providing structural integrity to the cable so that it will be not damaged by the substantial drag forces exerted upon the lengthy cable as it is towed through the water at speeds of several kilometers per hour. Commercial cables usually have three tensile members in the form of steel cables secured within the plastic tube at intervals of 120.degree.. Hydrophones are secured within the plastic tube, inside the cables and lying on the axis of the tube at regular intervals, typically about 1 m; these hydrophones incorporate pressure detectors, normally piezo-electric detectors, capable of detecting sound pressure in the water caused by the explosions used in seismic surveying. The hollow interior of the tube is filled with oil so that vibrations in the water surrounding the cable are efficiently transmitted to the hydrophones. Electrical conductors extend the full length of the hydrophone cable to supply power to the detectors and to carry signal from the detectors back to recording and/or analysis equipment carried on the ship. Signal conditioning modules are usually included approximately every 300 m for amplification and signal conditioning such as filtering, if required.
Although commercial hydrophone cables are normally towed at depths of about 6 to about 25 meters during seismic surveying, the hydrophones they carry may operate down to 100 meters or more. As will be apparent to those knowledgeable in anti-submarine warfare, in the absence of any special precautions, a commercial hydrophone cable of the type already described would make an excellent submarine-hunting device, and international sales of such cables would have to be regulated under munitions control regulations. To permit international sales of commercial hydrophone cables and certain other dual-use technologies without cumbersome regulations, the United States and thirty-two other countries have concluded the Wassenaar Agreement on Export Controls for Conventional Arms and Dual-Use Goods and Technologies. This Wassenaar Agreement, and the U.S. government regulations promulgated thereunder (see Commerce Control List, Part 774, Supplement No. 1, Category 6 - Sensors and Lasers) provide that hydrophone cables may be freely sold provided they are equipped with pressure-sensitive switches such that the hydrophones will cease to operate at depths exceeding 35 meters. This somewhat arbitrary limit is the average value of the depth of the thermocline present in deep ocean waters; to be useful in anti-submarine warfare, hydrophones must be capable of operating below the thermocline. Further, the commerce control list states that the pressure switches should not be adjustable once installed in the tube.
Providing a suitable form of pressure-sensitive switch to meet this "cut-out" requirement of the Wassenaar Agreement has proved difficult. Such a switch must be inexpensive. In practice, each of the thousands of individual hydrophones in an array needs its own switch (commercial users prefer to buy the hydrophone and the switch as an integrated unit, since installing separate hydrophones and switches in a cable is complicated and too expensive), and since the price for the integrated unit cannot exceed about $12, the cost of the switch must be very low. The pressure at which the switch closes cannot deviate substantially from the desired 35 meter setting, since in practice the hydrophones within each cable are arranged in sections of (typically) 96 further arranged in groups of 8 (typically), and premature closing of any one switch deactivates the entire group of hydrophones, so that premature closing of a few switches among the thousands in an array may deactivate so many hydrophones that the value of the survey may be greatly reduced, or the survey may even have to be suspended while the affected groups of hydrophones are replaced. With the costs of survey ships running into thousands of dollars per hour, such downtime is highly undesirable.
Cables are sometimes also immersed, accidentally or otherwise, more than 35 meters deep, and if the cable is no longer operational after such deep immersion, its replacement is costly, so the switch should also tolerate substantial over-pressure (i.e., it should be capable of being submerged substantially below 35, for example, 150 or more meters) without such over-pressure affecting the pressure at which the switch thereafter closes.
Vibrations from the water flowing past the cables are always a problem in seismic surveying. Since such vibrations appear as "noise" in the detected acoustic signals, it is undesirable for this noise problem to be compounded by vibrations caused by structures within the cable, and thus the in switch should, so far as possible, not transmit vibrations to the hydrophone.
In addition, it is desirable for any switch used with a hydrophone to not appreciably add to the overall volume of the combination since limited space is allocated for each hydrophone in an array assembly. Moreover, it is important to keep the hydrophone sensitive detection areas as far as possible from the noisy boundary layer at the external surface of the cable to enhance signal to noise ratios. Therefore, the switch should not alter any optimized hydrophone design that achieves this feature, and it is desirable for the switch to be acoustically isolated from the hydrophone and not alter its acoustic response characteristics.
Finally, although the cable is designed to surround the hydrophones with a non-corrosive oil, in practice sea water often leaks into a cable during extended commercial use, so the switch should be capable of resisting corrosion by salt water.
A typical prior art pressure-sensitive switch (generally designated 1) is illustrated in schematic cross-section in FIG. 1 of the accompanying drawings. This switch, which is of the so-called "dome" type, comprises a lower diaphragm 2 which is shaped to provide a circular elevated portion 3. The periphery of the lower diaphragm 2 is fixed within an annular insulating washer 4, through which passes an electrical conductor 5 extending from the lower diaphragm 2 to external circuitry (not shown). The switch 1 also comprises a dome-shaped upper diaphragm 6, the periphery of which is fixed within an annular insulating washer 7 which overlays and is secured to the washer 4. An electrical conductor 8 passes through the washer 7 and connects the upper diaphragm 6 to external circuitry. When the switch is exposed to atmospheric pressure, the upper diaphragm 6 stays in the position shown in continuous lines is FIG. 1, but as the pressure gradually increases at some point the "dome" of the upper diaphragm suddenly collapses to an essentially planar form, as shown in broken lines in FIG. 1, thus contacting the lower diaphragm 2 and electrically connecting the conductors 5 and 8 (the upper diaphragm 6 is of course exposed to the external pressure, while the lower diaphragm 2 is not).
Dome switches such as the switch 1 have difficulty in satisfying the aforementioned requirements for pressure-sensitive switches used in hydrophones. The pressure at which the upper diaphragm 6 collapses may be affected by minute variations in thickness or imperfections in material used to form this diaphragm, and hence substantial variations in closing pressure are experienced in batches of such switches. There is no easy method to adjust the closing pressure of individual switches. The switch 1 does not tolerate large over-pressures well since there is a limited area of contact between the upper diaphragm 6 and the narrow elevated portion 3 of the lower diaphragm 2 when the switch is closed. This means that under substantial over-pressure this limited area of contact is under great stress, and the upper diaphragm is likely to undergo non-elastic, permanent deformation, which changes the closing pressure of the switch after the over-pressure has been removed. It is difficult to make the switch 1 corrosion resistant, at least in part because the sudden sharp deformation which the upper diaphragm undergoes as the switch closes tends to crack any corrosion-resistant layer coated on to this upper diaphragm. To provide sufficient corrosion-resistance, it is conventional practice to embed the switch 1 within the same polymeric "potting" compound in which the hydrophone itself is typically embedded; however, this embedding of the switch increases the stiffness of the dome and substantially increases the pressure required to close the switch. Further, the potting compound may be affected by environmental changes such as temperature of the surrounding fluid, causing large variations in closure pressures over time. Finally, the switch 1 shows a tendency to fail mechanically by separation of the two washers 4 and 7, presumably because the sudden collapse of the upper diaphragm 6 as the switch closes exerts a strong radially outward force on the washer 7, thereby tending to cause it to separate from the washer 4.
The present invention seeks to provide a pressure-sensitive switch which is very suitable for use in a hydrophone cable (although it may be used in numerous other applications) and which reduces or eliminates the aforementioned disadvantages of prior art pressure sensitive switches. Preferred embodiments of the switch of the present invention allow adjustment of the closing pressure of individual switches at the time of manufacture. The switch of the present invention is especially useful in conjunction with hydrophones of the types described in U.S. Pat. No. 5,646,470 and 5,675,556, which are substantially cylindrical with openings at each end through which the external pressure is transmitted to the interior of the hydrophone.